The present invention relates to a vehicle load backup circuit and more particularly to a vehicle load backup circuit for use in control of the drive of a vehicle load by a CPU in which the vehicle load is backup driven when an abnormal state of the CPU is detected.
In recent years, while there has been increasing the tendency that a multiplicity of functions and highly improved functions are imparted to on-board electrical equipment (electronic units), the demand for smaller and more inexpensive electronic units has been getting more stringent. In order to realize these contradicting demands (or the demand for electronic units having functions equal to or better than those of related electronic units and made smaller in size and lower in cost), it is inevitable that the control circuits for controlling the vehicle loads need to be made smaller in size and lower in cost.
The vehicle loads include lamp systems such as headlamps and motors for operating the side windows in the electric power window system. In particular, the automatic headlight on/off control, which will be, hereinafter, referred to simply as an automatic light control, is a control in which the headlamp output circuit is automatically switched on or off not through a switch operation by the driver but through the use of programmed software based on information (dark or bright) from the automatic light sensor. The automatic headlight backup circuit, which will be, hereinafter, referred to simply as an automatic light backup circuit, is a backup circuit for detecting a malfunction or abnormal operation of the CPU to switch on the headlamp output circuit so that the headlamp output circuit is not switched off even in the event that something abnormal is generated in the CPU (even in the event that the control through software becomes impossible) in such a state that the headlamps are illuminated by the automatic light control.
FIG. 9 is a circuit diagram showing a related automatic light backup circuit disclosed in Patent Document 1. In this automatic light backup circuit 100, a pulse output terminal W100 is connected to a pulse input terminal W101 of a regulator, and an output from a CPU abnormality detection circuit 101 which includes resistors R101 to R106, capacitors C101 to C103, diodes D101, D102 and a NOR gate N101 and an output from a 5V power supply rising delay circuit 102 which includes resistors R107, R108, a capacitor C104 and a diode D103 are made to be an input of a NOR gate N102. In addition, in an automatic light backup circuit 100, an output from the NOR gate 102 is inputted into an emitter of a transistor T101 of an ignition switch (IGSW) signal circuit 103 which includes resistors R109 to R114, diodes D104, D105, a Zener diode Z101 and transistors T101, T102, and an output from a collector of the transistor T101 is inputted into a NOR gate N103. Furthermore, in the automatic light backup circuit 100, an output from an automatic light determination signal circuit 104 which is connected to an output terminal W103 of a backup from the CPU to which a resistor R115 is connected is inputted into the NOR gate N103. In addition, in the automatic light backup circuit 100, an output from the NOR gate N103 is inputted into a NOR gate N104 and an output from the NOR gate N104 is inputted into a gate of the transistor T103 via resistors R116, R117 (the resistor R116 being a pull-down resistor of the output from the NOR gate N104), whereby by switching the transistor T103 on and off, a relay for driving sidelights or small lights and a relay for driving headlights which are connected to the transistor T103 via diodes D106, 107, respectively, are switched on and off. Normally, a 5V power supply is used as a power supply for the four NOR gates N101, N102, N103, N104. In FIG. 9, the illustration of 5V power supply terminals to the NOR gates N101, N102, N103, N104 is omitted.
In the related automatic light backup circuit 100 that is configured as described above, when pulse signals outputted from the output terminal W100 are stopped, an abnormal state of the CPU is detected. Here, the operation of the automatic light backup circuit 100 will briefly be described. When the CPU operates normally, pulse signals are inputted into the terminal W100 from the CPU at predetermined time intervals, whereby HIGH and LOW output voltages are outputted repeatedly from the NOR gate N101 at predetermined time intervals. Here, a potential of the capacitor C103 is made to be a higher potential than a certain threshold by quick charging by way of the resistor R105 and delayed discharging by way of the resistor R104, when the CPU operates normally. A low-level voltage is outputted from the 5V power supply rising delay circuit 102 after a period of time which is determined by a time constant of the circuit has elapsed since the power supply is risen. Here, a logic circuit is configured so that a backup is not activated when the power supply is risen (when a high-level voltage is outputted). A low-level voltage is outputted from the NOR gate N102 after a predetermined period of time has elapsed since the power supply is risen. In addition, when an ignition switch IG1 or IG2 is switched on by the ignition switch signal circuit 103, the transistor T1 operates, and an output from the NOR gate N102 is inputted into the NOR gate N103. As this occurs, when the level of output from the automatic light determination signal circuit 104 is low, the level of output from the NOR gate N103 becomes high, while the level of output from the NR gate N104 becomes low, whereby the transistor T103 becomes inoperable. Therefore, the small lights relay and the headlights relay become inoperable, and hence, the small lights and the headlights are kept non-illuminated. On the other hand, when the level of output from the automatic light determination signal circuit 104 is high, the level of output from the NOR gate N103 becomes low, while the level of output from the NOR gate N104 becomes high, whereby the transistor T103 is put in an ON state, and the small lights relay and the headlights relay are put in operation, whereby the small lights and the headlights are put in an illuminated state. Thus, the above description is the operation of the automatic light backup circuit 100 when the CPU operates normally.
Next, the operation of the automatic light backup circuit 100 in a state that the CPU is abnormal will be described. When the CPU is abnormal, no pulse signal is outputted from the CPU, and no pulse signal is inputted from the terminal W100, whereby the capacitor C101 is discharged and the potential of the capacitor C1 constitutes a potential which is lower than the certain threshold. By this, the voltage level at the input terminal of the NOR gate N101 becomes low (L), and the voltage level at the output terminal thereof becomes high (H). Namely, the level of output from the CPU abnormality detection circuit is high. A low-level voltage is outputted from the 5V power supply risen delay circuit 102 after the period of time which is determined by the time constant of the circuit has elapsed since the power supply is risen. By this, a low-level voltage is outputted from the NOR gate 102 after the predetermined period of time has elapsed since the power supply is risen. In addition, when the ignition switch IG1 or IG2 is switched on by the ignition switch signal circuit 103, the transistor T1 is switched on, and an output from the NOR gate N102 is inputted into the NOR gate N103. As this occurs, when the level of output from the automatic light determination signal circuit 104 is low, the level of output from the NOR gate N103 becomes low, while the level of output from the NR gate N104 becomes high, whereby the transistor T103 is switched on, and the small lights relay and the headlights relay are activated to operate, the small lights and the headlights being thereby illuminated. On the other hand, when the level of output from the automatic light determination signal circuit 104 is high, the level of output from the NOR gate N103 becomes low, while the level of output from the NOR gate N104 becomes high, whereby the transistor T103 is put in the ON state, and the small lights relay and the headlights relay are activated to operate, the small lights and the headlights being thereby put in the illuminated state. Namely, when the CPU becomes abnormal, the small lights and the headlights are put in the illuminated state irrespective of the output from the automatic light determination signal circuit 104. Thus, the above description is the operation of the automatic light backup circuit 100 when the CPU becomes the abnormal state.
As has been described heretofore, the automatic light backup circuit 100 is the circuit configuration in which the four NOR gates N101, N102, N103 and N104 are used.    [Patent Document 1] JP-A-61-150040
In a vehicle load backup circuit like the automatic light backup circuit disclosed in Patent Document 1, since the configuration of the vehicle load backup circuit has logic ICs, those are the four NOR gates N101, N102, N103 and N104, it caused a problem that the control circuit is complex in circuit configuration and hence has to have a large number of parts involved.